The present invention relates to a control circuit for the switching of inductive loads, more particularly to a control circuit that can be integrated monolithically and which comprises a final class-B stage with push-pull transistors and which is usable for driving relays, solenoids, and DC motors. The simplest type of final class-B stage with push-pull transistors is composed of a pair of complementary transistors connected in cascade with the emitter terminals connected between the two poles of a D.C. supply voltage generator and the collector terminals connected together. The load is connected to the connection between the two transistors (i.e.-the collector terminals) which thus constitutes the output terminal of the stage. By means of their base terminals, the two transistors are alternately driven into conduction in phase opposition, providing current flow in opposite directions to the load.
However, the switching of such transistors does not occur instantaneously, but with a transient having a time duration which is not negligible, especially when the transistors conduct so as to be in saturation. Therefore, when they are simply controlled in phase opposition, during the switching of the final stage, they cannot be prevented from being turned-on simultaneously.
The simultaneous conduction of the two transistors generally causes an undesirable increase in the power dissipation in the final stage.
When the load connected to the final stage is of an inductive type, the back electromotive force induced by the variation of the current flowing therethrough, due to the switching of the final stage, suddenly increases the collector-emitter voltage of the transistor during the transition period when the transistor is being turned off but is still in conduction, thus causing in said transistor its maximum value of its power dissipation, sometimes with destructive effects.
Therefore, it is necessary to prevent the transistors of a final push-pull stage, including those used in a control circuit for the switching of inductive loads, from becoming simultaneously conductive during switching.
As is well known, the simplest way to resolve this problem is to provide a control circuit with a final push-pull stage, wherein the control for the turning on of the final turned-off transistor is appropriately delayed with respect to the control for turning off the conductive transistor.
FIG. 1 is a circuit diagram, partially in blocks, of this type of known control circuit which can be integrated monolithically. It comprises two bipolar transistors T.sub.1 and T.sub.2, of the PNP and NPN type respectively, connected in push-pull between the two poles +V.sub.CC and -V.sub.CC, of a DC supply voltage generator to which their emitters are connected, while their collectors are connected together so as to form the output terminal of the circuit.
The bases of T.sub.1 and T.sub.2 are each connected by means of a suitable biasing circuit, indicated in the FIG. 1 by the blocks P.sub.1 and P.sub.2, to the output of two comparators A.sub.1 and A.sub.2. The inverting input of A.sub.2 and the non-inverting input of A.sub.1 are connected together, constituting the input of the circuit to which is connected a switching-signal source denoted in the figure by the block SW. The inverting input of A.sub.1 is connected to the non-inverting input of A.sub.2 by means of a first constant-voltage generator V.sub.TH which is positive with respect to the potential of the non-inverting input of A.sub.2 ; the non-inverting input of A.sub.2 is connected to -V.sub.CC by means of a second constant-voltage generator V.sub.REF, which is positive with respect to the potential of -V.sub.CC.
With respect to -V.sub.CC, the input signal has a minimum level which is lower than V.sub.REF and a maximum level which is higher than V.sub.REF +V.sub.TH ; the transition of the input voltage from a value less than V.sub.REF to a value greater than V.sub.REF +V.sub.RG, or vice versa, determines the switching of the final stage.
When the signal level is lower than V.sub.REF, T.sub.1 is turned off and T.sub.2 is turned on. When the signal level is higher than V.sub.REF +V.sub.TH, T.sub.1 is turned on and T.sub.2 is turned off.
For signal values ranging from V.sub.REF to V.sub.REF +V.sub.TH, both transistors of the final stage are turned off. Therefore, by properly choosing the value V.sub.TH in accordance with variations in the switching signal time, one can rule out the simultaneous conduction of T.sub.1 and T.sub.2 or, if it is more convenient, limit the duration thereof so as to eliminate all destructive effects.
A monolithically integratable control circuit of the type under consideration effectively resolves the problem of any simultaneous conduction of the final transistors in a control circuit for switching with a final stage having push-pull output transistors, but uses for this purpose circuit means that are relatively complex, such as the two comparators, and are therefore costly in terms of integration space.